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WADC TECHNICAL REPORT 54-371r " SUPPLEMIENT 1

SA "TIA DOCUMENT No. AD 97301

INVESTIGATIONS OF RHENIUM

CHESTER T. SIMS GOt1DON B (.AINESCtARLES M. CRAIGHEAD FRANCIS C. TODD

ROBERtT I, JAFFEE CIHARLES S. 'EFETDONALD N. GIDE(ON DANNY M, RO SEVNIJAUM

WILIJU.: W, KLEINSCTMIDT BO)GER I. I1UNCKWILLIAM E. NEXSEN, ]Il. IVOR E, CAMPBELL

BATTELLE MEMORIAL INSTITUTE

SEPTEMBER 1956

WRIGHT AIR DEVELOPMENT CENTER

WADC TECHNICAL REPORT 54-371

SUPPLEMENT 1

ASTIA DOCUMENT No. AD 97301

~ R INVESTIGATIONS OF RHENIUM

CHESTER T. SIMS GORDON B. GAINES

CHARLES M. CRAIGHEAD FRANCIS C. TODD

ROBERT I. JAFFEE CHARLES S. PEET

DONALD N. GIDEON DANNY M. ROSENBAUM

WILBUR W. KLEINSCHMIDT ROGER J. RUNCK

WILLIAM E. NEXSEN, JR. IVOR E. CAMPBELL,

BATTELLE MEMORIAL INSTITUTE

t SEPTEMBER 1956

AERONAUTICAL RESEARCH LA.BORATORY

CONTRACT AF 33(616)-Z32PROJECT 7080

TASK 7065i9

ARWRIGHT AIR DEVELOPMENT CENTERARRESEARCH AND DEVELOPMENT COMMAND

UNITED STATES AIR FORGE

WRIGHT- PATTERSON AIR FORCE BASE, OHIO

rpn Litho, & P±-tg. Co., Springfield, 0.iO -Ocober 1956

FOREWORD

This report has been prepared by Battelle Memorial Institute underUSAF C-ontract No. AF 33(l16)-23, "Invcotigationo of Rhenium". Thepresent report is the final technical report on. the project, and covers thesecond twoo-year period of work on this project, from June, 1954, toJune 30, 1956. A previous WADC Technical Report, No. 54-371, describesinvestigations carried out during the period from June, 1952, to June, 1954.The contract was initiated under Project No. 7080, Task No. 70659, andadministered under the direction of the Aeronautical Research Laboratory,Directorate of Research, Wright Air Dovelopiuieni Center. pkroject engineersduring -the period covered by this report were Lt, John P. Hirth and Mr.J[ames Poynter.

The Battelle worl as divided among three research groups. Prepa-ration of rhenium metal iowder, and vapor presoure and electroplatingstudies were conducted by D. M. Rosenbaum, Principal Chemist, R. J.Runck, Assistant Division Chief, and I. E. Campbell, Chief, of the InorganicChemistry and Chemical Engineering Division. Physical, mechanical, andmetallurgical properties determinations were studied by C. T. Sims,Assistant Division Chief, C. M. Craighead, Division Consultant (d,-ceased),and R,. I. Jaffee, Chief, Nonferrous Physical Metallurgy Division. W. W.Kleinschmidt, Laboratory Technician, D. N. Gideon and W. E. Nexsen,Principal Physicists, G. B. Gaines, Assistant Chief, and F. C. Todd andC. S. Peet, Chiefs of the Electronic Physics and Solid State DevicesDivisions, respectively, conducted studies of physical and electronicproperties.

Potassium perrhenate for chemical and metallurgical processing intorhenium metal was kindly donated by the Kennecott Copper Corporationthrough its subsidiary, The Chase Brass and Copper Company.

I

ABSTRACT

A new method It reported for the preparation of high-purity rhenium metal powder byreduction of a hydrolyzed rhenium halide. Fabrication and consolidation of thitc material isdiscussed. The effect of thoria #dditions on rhenium fabricability are evaluated, ind additionalinformation on the hot and cold working of pure rhenium given.

The electrical resistivity and specific heat of rhenium at room and elevated temperaturesup to 2700 K are reported in detail. The electromotive forces generated by Re-Pt, Re-W,Re-Mo, and Re-Ta thermocouples were studied and the results are given.

Mechanical properties of several types of fabricated rhenium were measured and arediscussed. These include tensile properties of annealed and cold-worked 10-mit stri; ntv-,f.-ruipture charavt't-ri-ticz of 50-ici wire, wo-k-hardening studies on rod, wire, sheet, and foil.The shear modulus of elasticity is reported and the temperature dependency at the modulus ofelasticity up to 900 C given.

2.lect7onic studies reported include results of the effect of additlon& of 2. 0 per cent thn-rium E.s ThOZ on the thermionic emission and an evaluation of the photoelectric work functionof pure rheiuin-. The stability of rhenium and tungsten filaments in contact with alumina at1600 C, and the stability of rhenium, tungaten, and molybdenum in carbonaceous atmosphereswere evaluated and the results are discussed. Rhenium and tungsten filaments were studied forresistance to thermal and mechanical shock.

The resiatance of rhenium to attack by molten metals was also evaluated and a discussionis included,

Arc-melted platinum-rhenium alloys containing up to 10 per cent rhenium were sgidiedfor fabricability and properties. A successful method for fabricating alloys containing up to2. 0 per cent rhenium was developed and is discussed, and the results of resistivity, thermalernf, tensile streangth and ductility, hardness, and oxidation resistance determinations en thesealloys are given.

The report also includes results of several investigations conducted outside Battelle atindustrial or educational research establishments with rhenium supplied by this project.

PUBLICATION REVIEW

This report has been reviewed and is approved.

FOR THE COMMANDER:

NATHAN L. KISERG "Colonel, USAF UChief, Aeronautical Research LaboratoryDirectorate of Research

WADC TR 54-371 Suppl 1 iii

TABLE OF CONTENTS

Page

INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . I

METAL PREPARATION, CONSOLIDATION, AND FABRICATION ................................... 3

P Rheniu m Metal P eom.Ao.monium Perrhenate ..................... ...................... 3Prprto fRhenium Metal Fo APionmderrhnt......................................... 3

Rhenium Metal by the Halide Method ........................ .......................... 5Fab;rication of Halide-Process Rhenium. ........................ . ......................... 9Fabrication of Perrhenate-Process Rhenium.. ................ .... ......................... 9

Hot Swaging ........................ ...................................... 10Coli Rolling .... . ................................ ............................... 10Fine Wire Drawing.. .............................. ................................. 10

Fabrication Comparison of Thoriated and Nonthoriated Rhenium ..... ................ .. 11

PHYSICAL I ROPERTIES ............... ......................................... i

Electrical Resistivity ............ .. . .. . ............................. 12Preparation of Resistivity Specimen ... .. ........................ 12Resistivity at Room Temperature ... .................. . . ............... 12Resistivity From Room Temperature to 72- K (452 C) .............. .................. 15

Resistivity From 629 K (356 C) to 1175 K (902 () ..... .. .................... 1. 17Resistivity From 1100 K (827 C) to 2050 K (1770 C).. .. ..... ................... .. 17Resistivity From 2000 K (1127 C) to 270C F (2427 C) ........ .................. 19

Summary of the Resistivity Data ..................... .............................. . . 1

Temperature Coefficient of Resistivity ................... ........................... . 23Specific Heat of Rhenium. ...................... ................................. 23

Vapor Pressure .......................... ..................................... 34Electromotive Force Studies ................ .......................... .. ... .. . 34

Rhenium vs Platinum ........................ ................................ 34Rhenium vs Tungsten, Molybdenum, and Tantalum.. ....... ................... .. 35

MECHANICAL PROPERTIES ......................... .................................. 40

Teusile Properties of Cold-Rolled Sheet ................ ........................... .. 40Work-Hardenine Ch- acteristics of Rod, Sheet, and Wire ........ ................... .. 43Shear Modulus of Elasticity .............................................. . . . 43Modulus of Elasticity Variation With Temperature ............. ...................... .. 45

Stress-Rupture Properties ...................... ................................. .. 45

ELECTRONIC PROPERTIES ................................................... ........... 49

Thernionic E•nission of Thoriated Rhenium.. ............. ......................... 49The Photoelectric Threshold of Rhenium ................. ........................ ... 52

MISCELLANEOUS PROPERTIES ........................ ................................ 57

Stability to the Water Cycle Reaction ..................................................... 51.

Stability to Attack by Aluminumn Oxide ................ ............................ .. 58The Stability of Rhenium, Tungsten, and Molybdenum in Contact With Carbon-BearingAtmospheres ........... ............................... ............. . 58

Stability to Solution by Molten Metals ............. .... ......................... ... 61

STABILITY OF RHENIUM AND TUNGSTEN SUBJECTED TO MECHANICAL AND THERMAL SHOCK . 62

PROPERTIES OF PLATINUM-R-ENIUM ALLOYS .............. ......................... 64

Fabrication of Platinum-Rhenium Alloys . . . ..................... 64

Electricalý Resistivity of Pt.-tintmn-R-henia,, .L& . .................... 65Thermal Electromotive Forces Produced by Platiroui. vs Platinum-Rhenium Alloys ........ 65Mechanical .Propertiet of Platinum-Rheniun -Alloys ....... ..................... 67

Teumile Strength and Ducrility ......................... ........... ................. 67Harnius . ........................... ...................................... 69

O.J-ation Iesistance of Platinum-Rhenium Alloys ............................ 69

WADC TR 54-371 Suppl 1 iv

TABLE OF CONTENTS

(Continued)__

Page

DISTRIBUTION OF INFORMATION AND MATERIALS ..................... . . 71

Organizations Contacted .. . . . . .. .. .. . .. .. . .. .. .. . 71Research Reported by Various aducational %nd Industrial Organizations. ............. 73

Wear Resistance ........ ............................ 73Field Emission ............ ................................. 73

Electron Ejection Target .?...................................................... 73Vacuum Lamps ....................... ................................... .. 74X-Ray Micro-Focus Tube .................... ............................... .... 74TR Switching Tubes ....................... ................................. ... 74Glass-to-Metal Seals and Machinability .. ......... . ........ ................ ... 74Corrosion Resistance .............. ................ ........... 74Gauge Tube ........ : . : ................... 74Stylus Material ............................. ................................... 75

Evaporation Filament ....................... ................................ ... 75* Spectral Eniasivity. . ..... ............ I... . . . . . . . . . . . . .. . . . . .. .75

SUM-MARY ................... ................. .......................................... 75

REFE RENCES . ................................... ......................................... 78

LIST OF TABLES

Table 1. Table of Analyses ........................ .................................... 8

Table 2. Spectrochernical Analysis of Rhenium Resistivity Specimen ...... ................ ... 13

Table 3. The Electrical Resistivity of Rhenium Metal From 20 to 2400 C .......... ............. 25

Table 4. Temperature Coefficients of Resistivity Calculated From the Present Work and Compared

to Those of Agte, et al ............................ ............................. . .2

Table 5. Tensile and Elongation Data for Annealed and Ccld-Rolled Rhenium Sheet ........... ... 41

Table 6. Stress-Rupture Characteristics of 50-Mil Rhenium Wire at 1000 C .... ............ .. 47

Table 7. Attack of Rhenium by Molten Metals ........ ....... .......................... ... 61

Table 8. Mechanical Properties of Platinum and Platinum-Rhenium Alloys .... ............. ... 67

Table 9. Oxidation of Platinum and Platinurn-lutenium Alloys in Air at 800 C. I .. ......... .. 69

LIST OF ILLUSTRATIONS

Figure 1. Scheme for the Preparation of Rhenium Metal by Hydrolysis of a Volatile Halide .... ...... 6

Figure 2, Apparatus for Chlorination of Rhenium .......................... 7

Figure 3. Schematic Drawing of the Vacuum Chamber Employed for the Rhe:nium ResistivityMeasurements. ............ ................................ . ..... 14

Figure 4. Electrical Resistivity of Rhenium From Room Temperature to 1273 K (10-00 C) ......... ... 16

Figure 5. Schematic Drawine of EIrtýrl•.- .... . . -, ! •-t<of Rhenium. ............................. ..................................... 18

Figure 6. Electrical Resistivity of Rhenium From 1100 K (827 C) to 2700 1.C (Z427 C) ..... ......... 20

Figure 7. Average Plot of the Data Relating Elect,icai Resistivity of Rheniurn to Tenperature . . . . 22

Figure 8. Electrical Resistivity of Rheniurn as a Function of Temperature ..... . 4

WADC TR 54-371 Suppi I v

LIST OF ILLUSTRATIONS(Continued)

Figure 9. Diagram of Equipment for Specific-Heat Measurements . 28

Figure 10. A Representative Response of the Specific-Heat Apparatus to a Step Increase in Current . 30

Figure 11. Relationship Between Current Through Rhenium Wire and 'rime, Showing a TypicalStep Increaee of the Current ........................... ............................. 3i

Figure 12. Specific Heat of Rhenium Ut High Temperatures .................... 32

Figure 13. Comparison of Specific Heats of Rhenium and Tangroten . ...... ................ .. 33

Figure 14. Experimental Points and Calculated Curve of the Thermoelectric Force Produced in .Platinum- Rheniurn Thermocomple ...................... ........................... 36

I igure !. Electiuouiuive Farce Versus Temperature for C~rtain Metals With Respect to Platinum . . 37

Figure 16. Experimental Data and Matching Curves Calcalatcd !or the Thermal Electromotive ForceProduced by Rhenium-Molybdenum and Rheniurn-Tungsten Thermocouples ........ .. 39

Figure 17. Tensile Properties of Annealed and of Coll-Rolled Rhenium Sheet .... . ...... ....... 42

Figure 18. Cold Work-Hardening Characteristics for Various Types of Rhenium Compared WithThose of Nickel .................................. .................................. 14

Figure 19. Variation of the Modulus of Elasticity of Rheniurm With Temperature ............. .. 46

Figure 20. Parametric Plot af Tensile and Ru.pture Data for Pure Rhenium Metal ....... .......... 48

Figure 21. Schottky Slopes for Thoriated Rhenium Wire ............. ...................... .. 50

Figure 22. Richardson Plot for Thoriated Rhenium Wire .......... ...................... .. 51

Figure 23. Experimental Phototube for Photoelectric-Threshold Measurements ....... ............ 53

Figure 24. Diagrammatical Representation of Apparatus for the Determination of PhotoelectricWork Function ............. ............ ................................... 54

Figure 25. The Appearance of Rhenium -1' Tungsten After Contact With Alumina ior 7000 Hournat a Brightness Temperature of 1600 C ............. ........................ 59

'igure 26., The Ch-anmbcr for Testing the Re i; co of RhP•,uzn to Attack by A1203.. . .... ......... 60

Figure Z7. Mounting Arranmgement of Rhre:iium und Yungsten Wires for Vibration Tests ..... ........ 63

Figtgre 2•. Electrical Resistivity of Several Platinum Alloys ........ ..................... .. 66

Figure 29. Thermal Electromotive Force Data for Pt-1. ORe and Pt-Z. ORe Versus Platinum•........ 68

Figure 30. The Effect of Rhenium and Other Metals on the Hardness of Platinum ... ........... . 70

WADC TR 54-371 Suppl I yj

INVESTIGATIONS OF RHENIUM

INTRODUC TION

This investigation of rhenium has been concluded after four years ofexperimental work. The first two years' work was summarized in WADCTR 54-371, dated June, 1954. The present final technical report coversthe period from June, .1954, to June, 1956.

I._N.• A • sI re7%orred results wA-ich were thc basis for the ex-

perimental work discussed herein. Rhenium is there described as a dense,hard, high-melting metal, difficult to prepare and consolidate, and fabri-cable only by cold-working and annealing. It was found to have excellentroom- and elevated-temperature strengtk,> ýi-d in many instances behavedmuch like tungsten, as might be expected 1-ým its position in the periodicsystem. However, rhenium differs from tungsten in several importantrespects. For example, it waG found to retain ductility a•ter thermalcycling above its recrystallization temperature and to be highly resistant tothe water cycle. Under certain circumstances it appeared to be a morepromising electrical contact material than tungsten, although not as good athermionic emitter. Many of the results obtained suggested that rheniumhad an excellent future in electronics and allied fields. However, manyimportant properties needed further study and some had not been investi-gated at all, so that the experimental work reported here is a direct con-tinuation of the work reported in WADC TR 34-371.

During the last two years of research a new method, which is probablysuperior to the ammonium perrhenate method, was developed for the prep-araiion of rhenium metal powdpr. This is the halide process.

As during the first two years, studies of the basic physical, me-chanical, and electronic properties of rhenium formed the core of thesecond half of the program. The electrical resistivity of rhenium wasredetermined and firraly established from room temperature to about2300 C. The specific heat was also studied over this range, and the previousvapor pressure work was reviewed. The thermal ernf behavior of Re-Ptalloys was evaluated as was that of rheMiirn r-1ipled with more refractorymetals mentioned below. Tensile properties of thoriated rhenium, rheniu.sheet, and rhenium-platinum alloys wcr'e evaluated, and the shear modulusand elasticity modulus of pure rheniur... .'e studied, Electron erissionby heating thoriated rhenium and by use of the photoelectric threshold ofpure rhenium, was also studied.

ý4nuscpiipt relvaied by the authors in June 1956 !or publication as a WAD•(Technical Report.

WADC TR 54-371 Suppl 1 1

Some of the properties investigated were pointed directly towardfuture industrial use of rheniurn. For instance, comparison of known datawith the thermal emf generated by the rhenium-platinuw thermocoupleshowed that rhenium-molybdenum and rheniumn-tungsten thermocouplesshould also generate high enifs. The work was confirmed experimentally,and it was also found that the couples generally show a linear increase withincrease in temperature. Thermocouples of these elements, or possiblyrhenium alloys, should be very interesting, although much development workstill remains, Use of rhenium in the electronics field was pointed up byresults of a test comparing the stability of rhenium and tungsten wires incontact with AlZ0 3 at 1600 C in vacuum. Both metals lasted over 7000 hoursto the end of the test, but the tiingsten was qeriog1.v -=Itt;IrePd by watercycling. It was also verified that rhenium does not form a carbide. Whenrhenium and tungsten wires were exposed to a carbonaceous atmosphere,the tdngsten became brittle because of carbide formation, while the rheniurmappeared to retain its original ductility.

Rhenium shows good resistance to attack by molten metals except foriron and nickel. Nevertheless, one industrial concern is utilizing rheniumas an evaporation filament for silver and nickel. Results of other industrialwork with rhenium are also given in this report.

Most of the properties obtained indicate that rhenium is a metal ofconsiderable interest, with a promising future. However, its scarcity andhigh cos. dictate that ary application of the metal must meet two conditions:

(1) The application must require only smnall quantities ofrhenium per unit item.

(2) Rhenium, must be critical in the application. That is,it must perform appreciably better than competingmaterials.

The use of rheniumn in electronics and in electrical contacts fits thescerequirements very well. Development work in the latter field, initiated byinformation reported in WADG TR 54-371, has been taken up by P. R.Mallory & Co. of Indianapolis, indiana.

The Chase Brass and Copper Company, which kindly furnished therhenium used in these investigations, has been developing co' Iercimethods of producing the metal in the common fabricated forms for use inthe electrical and electronics fields. Thus the metal appears to be welllaunched as a new material of technology. However, much more researchand development is needed before rhenium's potential usefulness is realized.

WADC TR 54.-371 SuppI I j

METAL PREPARATION0 CONSOLIDATION, AND FABRICATION

Prejparation of Rhenium Metal Powder

The main objective of this phase of the investigation was to preparehigh-purity rhenium powder for subsequent consolidation by powder metal-lurgy methods. It had been previously determined that rhenium powder pre-pared by the hydrogen reduction of potassium perrhenate was unsuited forfabrication because of the presence of residual potassium oxide. Con-version of potassium perrhenate to ammonium pe:rrhenate(l). fnllnouwpd by,

hydrogen reduction of the ammonium perrhenate, yielded a powder whichwas very low in potassium and which could be successfully pressed andsintered. This method of preparing rhenium metal powder was investigatedfurther, and other new methods were inv,,stigated also. The sectionsimmediately below describe the ammonium perrhenate rm:ethod, variousgrinding and purifying procedures investigated, and the best method evolved,the halide method of producing rhenium metal.

Rhenium Metal From AmmoniumPerrhenate

One difficulty of the ammonium perrhenate method is the necessityof grinding the crystalline ammonium perrhenate to a particle size of lessthan 325 mesh in order to produce metal powder fine enough for sintering.The comrninution is carried out in a rubber-li-ned ball mill using "Burundum"balls. The initial itmfurity pickup from this grinding operation is low but,after the Burundu tn~a balls became slightly worn, the ammonium perrhenatevicks up approximately 0. 15 per cent i"mpurities Uduring griding. These-impurities apparently reduce the sinterability of the resulting rheniummetal powder. Other methods, both chemical and mechauical, for reducing

the particle size of purified ammnonium perrhenate were therefore investi-gated.

The first chemical method consisted of precipitating ammoniumperirhenate from saturated aqueous solutions by lowering the solubility ofammonium perrhenate through the addition of soluble organic liquids, in-cluding methyl alcohol, e.•thyl alcohol, and acetone, This process was triedat temperatures of 25 C and 4 C. At 25 C, the solution became milky incolor but the crystals were so fine that none settled out. At 4 C, precipita-tion of some coarse ammonium perrhenate (about ZOO mesh) occurred. Thiswork indicated tlat precipitation of ammonium perrhenate of the desiredparticle size probably could be accomplished by this method at a temperature

(a) Approximately 91% A12 03 ; balance CaO, 0203 and MgO.

WADC TR 54-371 Suppl 1 3

between 4 and 25 C, but that recovery would be very low. The maximu-nrecovery was only about 10 per cent, obtained at the lowest temperaturetwhere the ammonium perrhenate was too coarse to be satisfactory.

Improvement in the mechanical methods of reducing the particle sizeof purified ammonium perrhenate was attempted by modifying the equip-ment used for ball milling. Tungsten carbide rods were substituted for theBurundum balls used previously. A small l-quart rubber-lined ball millwas charged with about 100 1/2 by 1/2-inch rods and 100 1/4 by 1/4-inchrods of tungsten carbide. This charge of rods was tumbled in the mill in asolution of commercial detergent for 5 days in order to round off the sharpedges of the rods and to remove irregularities in the rubber lining. Then

.P gram .. o .f c "rybLaline amazonium perrhenate was ground in tie mill for Ihour to break it in. The fine nerrhenate -RS removed and reprocessed.

After this preparatory work, one pound of ammonium perrhenate wasground in the mill for I-'/2 hours. Almost all t&e resulting perrhenate wasof the desired particle size (-325 mesh) and the ntire lot of material wasused without screening. An analysis of the fine ammonium perrhenate(see Table 1) indicated the presence of 0.25 per cent tungsten. While thepickup of tungsten from tungsten carbide rods was of the same order ofmagnitude as the pickup of impurities from Burundum balls, the tungstencarbide impurity had little noticeable effect on the densification of the powderduring sintering operations. It was expected that with additional use of themill the tungsten carbide impurity in the ground perrhenate would decrease.

No further work was done by this method, however, because, forreasons not determined, th1e•, potassium content in the ammonium perrhenateprogressively increased with succeeding batches to a value of 0.47 per cent.When this perrhenate was reduced to the metal, it contained 0. 40 per centpotassium, approximately the same amount as that found in metal producedby the hydrogen reduction of potassium perrhenate. As previously stated)the fabrication by powder metallurgy techniques of rhenium powder pre-pared from potassium perrhenate was unsuccessful.

Two different methods of purifying this metal were tried: (1) leachingwith dilute hydrochloric acid followed by leaching with distilled water, and(2) conversion of the metal to the pentachioride, followed by hydrolysis ofthe chloride to produce rheniuin dioxide. Leaching was not effective infurther purifying this metal. However. the chloride or halide method,

which is described in detail in the following sectionj produced high-purityoxide from which high-purity metal was obtained by hydrogen reduction.

A.D,* TR 54-371 Suppi 1 4

Rhenium Metal by theHalide Method

Rhenium metal obtained from potassium.. perrhenate, a, onji-n

perrhenate, or scrap massive rhenium (see Figure 1) was placed in a Vycorreactor 2 inches I. 1). x 6 inches long (see Figure 2) and heated to 1000 Cin a hydrogen atmosphere to assure that all the rhenium was reduced to themetal. The reactor was cooled to 750 C in a helium purge and held at thistemperature for I hour in order to assure the complete removal of anyhydrogen. The helium purge was shut off and sufficient tank chlorine waspassed through the charge to keep a slight positive pressure on the exit line.Tlhe chiu• lut was Conu-Lksthd in au air-uooLedL trap. Under these conditions,rhenium metal was chlorinated to rhenium pentachloride at the rate ofabout 150 grams per hour.

When the chlorination was completed, the chloride was hydrolyzed byadding it cautiously to distilled water coolnd to about 10 C in an ice bath.The main product of the hydrolysis was finely divided hydrated rheniumdioxide. Perrhenic acid and chlororhenic acid were also formed in smalleramounts. In cold water, approximately 75 per cent of the available rhenliumwas recovered as rhenium dioxide. If the hydrolysis is not carried out incold water) the yield of rhenium dioxide is somewhat lower. The hydratedoxide had the cciisistency of a gel and was quite difficult to filter. Howeverjbubbling carbon dioxide through the hydrolysis products for approximately1/2 hour reduced the filtration time by as much as 94 per cent.

The hydrated rhenium dioxide was filtered off or separated from theliquor by centrifuging, washed several times with distilled water, and driedin a vacuum desiccator.

The bulk of the rhenium, as ReOZ; was reduced to metal by hydrogen)by exposing a 1/2-inch layer of ReO2 in a molybdenum boat to hydrogen forone hour at 400 C and for a second hour at 600 C. The rhenium metal pro-duced was loosely sintered and was broken up in an agate mortar and againtreated with hydrogen at 800 C for two hours. The charge was then cooled

to room temperature in an inert or reducing atmosphere.

To recover the rhenium from the filtrate, a small amo-unt of 30%hydrogen peroxide was added to the solution to convert all the rheniumpresent to perrhenic acid. The perrhenic acid was neutralized by theaddition of arm-nonium hydroxide and the resultant ammonium perrhenatefiltered off And reduced to the metal by hydrogen.

A typic.al analysis of rhenium powder prepared by the halide processis given in Table 1 along with the analysis of rhenium prepared f:rompotassium perrhenate and ammonium perrhenate. The over-all :rhcniumrecovery efficiency was about 95 per cent.

5WADC TR 54-371 Suppl 1 5 .

rRhgnium Metal(i) From reduction of potassium perrhonate(2) Scrap from fabrication

(3) Chemical recovery

HZO Reduction, 1000C LPure dry Ht

Re + Fa + SiOg

Chlorination, 750-8500 C Tank Clg

ReCId +FeCi_,

Hydrolysis H20 ( 10c)

FeCi3 +RoQ*+HNO-- H*IRGCl9+ H20

Fe CJ 1.

HReO 4 Filtration"HgReCI6

(to recovery) Re 06+ H.0

Drying"- Vucuum desiccator

ReOp

HO Reduction, 6005C Pure dry "I,

Re powder[Fe. 0,012 %; Si, 0.150i.-5-•

N- 25304FIGURE 1. SCHEME FOR THE PREPARATION OF RHENIUM METAL

BY HYDROLYSIS OF A VOLATILE 1-ALIDE IWADC TR 54-371 Suppl 1 6 g,i

Outlet to aoir-

cooi-Td 1 rap

r

Inverted -test tube K

k x× X xRhenlurn chargekxxx lxxx

x x x x xx1xxX x x xx x

xxx x xx x

Chlorine

L Hydrogen or helium

FIGURE 2. APPARATUS FOR CHLORINATION OF RHENIUM

WADO TR 54-371 Siipp). 1 7

TABLE 1. TABLE OF ANALYSES

Weight Per Cent

Rhenium Metal Rhenium Metal Rhenium Metal Rhenium MetalOrepared Fron Pre~paed From Prepared From Prepared By

Potassium Ammonium Atmmonium The Halide

Ele me nt Perrhenate Perrhenatc(1) Perrhenate(2 ) B& thod

Aluminum 0. 009 0. 094 0. 002 0. 008

Calcium 0. 008 0.017 0. 005 0. 002

Chromium 0.004 N.F.* N.F. N.F.

Copper 0.0012 0. 0021 0. 0008 0. 0002

Iron 0.05 0. 024 0.010 0.012

Magnesium 0. 002 0. 038 0. 003 0. 005

Manganese 0. 007 0. 002 N. F. 0. 007

Molybdenum 0.15 N.F. N.F. N.F.

Nickel 0.003 N.F. 0.002 N.F.

Potassium 0.41 N. F_ (0. 40)? N.F.

Rhenium Major Mitjor Major Major

Silicon 0. 005 0. 028 0. 125 0. 015

Sodium 0.15 N.F. N.F. N. F.

Tungsten N.F. N.F. 0.25 N.F.

Total 0. 799 0. 205 0.398 0. 049

*N. F. - Not found.

(1) Ground with "Burundum" balls.(2) GrQ11rd with tungsten carbide rods.(3) The value of potassium listed is a result of an impurity in the ammonium pe-r'ienate and was not an impurity

picked up during grinding operations. This value gradually increased with subsequent batches of ammonium

parthenate to the reported value. The halide method was ýhen used for the preparation of rhenium metal.The, potassium content of 0.40 per cent is not included in the total impurity value of 0. 398 per cent.

Elements checked for but not found: antimony, arsenic. barium, beryllium, bismiLb, boron, cadmium.cobalt, columbium, gallium, germanium, gold. lead, platinum, silver, strontium. tellurium, tin, titanium,vanadium, zinc, and zirconium,

WADC TR 54-371 Suppl 1 8

The halide process has several advantages over the other processes:

(1) The metal prepared is higher in purity than thiat ob-tained by the other processes; in particular, it isfree of potassium.

(2) The particle size of the metal powder prepared bythe halide method is such that it can be sinteredsatisfactorily and no comminution is necessary.

(3) The halide method involves substantially fewer stepsthan the ammonium perrhenate method.

Details of the fabrication of halide metal are discussed in the followingpart of this report.

Fabrication of H-alide-Process Rhenium

Rhenium metal powder prepared by the halide process was consolidatedand worked successfully to rod, wire, and sheet. The metal was pressedsintered, and fabricated by the processes described in detail previously(1$.The halide-process rhenium metal powder pressed to more dense compactsthan the metal powder produced by hydrogen reduction of aniimonium per-rhenate, but did not sinter to as high density. It was previously determinedthat the perrhenate-type metal can be pressed (at 30 tsi) to densities ofabout 40 per cent of ideal density, and sintered (at 2700 C) to 90 per cent ofideal density. The halide-process rhenium can be pressed to 60 per centbut sintered to only 80 per cent of ideal density under the same conditions.Although a sintered density of about 90-.95 per cent of ideal is desirable: thehalide-process metal is still capable of fabrication to sheet and wire withcareful handling.

Further study of rhenium prepared by the halide process is needed,since the metal prepared by this process is of higher purity and the prepara-tion scheme is less costly than in the earlier processes. The poor densifi-cation achieved by hydrogen sintering is probably abnormal and due to acontrollable secondary factor such as excessive surface oxide on the metalpow.Hde r particles.

Fabrication of Perrhenate-Process Rhenium

During the two years' work covered by this report, several pounds ofrhenium metal in rod, wirc sheet, and foil form weie pi--±pa-cd by reduction

WADC TR 54-371 Suppl 1 9

of ammoniumn perrhenate. The procedures used to fabricate this metalhave been described in detail previously(1): aiid involve cold working andannealing of metal bars consolidated by powder metallurgy techniques.Rhenium can not be hot worked because of hot shortness. However: thecold-working and annealing procedure for reduction of rhenium to finalfabricated form is quite laborious and time-consuming, so that severalshort investigations were undertaken to attempt to improve the situation.These studies are discussed belowj together with the results of fabricationstudies on the preparation of fine wire and foil.

Hot Swaging

A specimen of high-purity rhenium containning about 0. 09 per centtotal impurities(a) was swaged hot in air from ahydrogen-atmospherefurnaceto establish whether its lower impurity content would result in improved hotworkability. The hydrogen furnace temperature was 1700 C. Four swagingpasses were made, each at a reduction in cross-sectional area of 10 percent. After the first pass, a longitudinal crack appeared, After the secondpass, the surface hardness was nearly 700 VHN; therefore a 2-hour annealat 1700 C was given. After the fourth pass, the bar was badly cracked andthe experiment was halted. It was obvious that rhenium of improved purityis also hot short. No further attempts to hot swage rhenium were maude.

C old Rollin.

Previous data(l) show that it is possible to reduce sintered rhenium tostrip about 8 mils' thickness in a two-high rolling mill with rolls 3-1/2inches in diameter and 6 inches long. Reduction was of the order of only0. 001 inch per pass when a thickness of 10 mils was reached, and consider-able intermediate annealing was necessary to secure reductio:n to 8 mils.

During the present report period, further rolling was done in a four-high Waterbury-Farrell semiautomatic foil mill with 2-inch work and 6-inch backup rrdls, Reduction was much mnore rapid in this mill, and, inonly 5 to 12 passes, 5- and 3. 5-mil strip foil was prepared from the 8-milstock. The metal was flat, had an excellent surface, and was almost withoutedge cracks in the finished condition. No reduction below 3. 5 znails wasattempted.

Fine Wire Drawing

Previously, no attempt had been made to prepare wire under 10 nnils

in diameter. However, demand by industry for samples of sub-0.-mil sizes(a) iRhenurn prepared In this work normally contained about 0.2 per cent total 1mpuritte% (see Table 1).

WADC TR 54-371 Suppl 1 10

was persistent, and it was felt that the finer stock should be prepared. Thiswould also establish the feasibility of fine wire drawing in general.

Rhenium containing 0. 5 per cent thorial previously prepared forelectronic studies and available at 20 mils diameter, was used for this pur-pose. It was found that below 15 mils diameter two 10 per cent reductionscould be taken between anneals. Diamond .dies were used from 10 mils downto 2. 9 mils, the smallest diameter of wire prepared. Houghton' s Cyl-TalNo. 81 was used as drawing lubricant. The 2, 9-mil wire appeared satis-factory, although microinspection indicated some surface pitting. Dies arenot available for drawing below 2. 8 mils, but it is believed that finer sizescould have been prepared.

Fabrication Comparison of Thoriated andNonthoriated Rhenium

Recrystallization data on thoriated rhenium(l) indicate that thorialowers the recrystallization temperature. Laboratory experience also in-dicates thoriated rhenium is annealed much more readily than pure rhenium.Therefore, it was decided to conduct studies on the use of thoria to improvefabrication. 0. 75 per cent thorim'n was uniformly dispersed in rheniumpowder by adding a water solution of Th(N03)4, converting to Th(OH)4 bydropwise addition of NH4OH, drying, and sintering. During sintering) theTh(OH)4 was in all likelihood converted to ThOZ. Next, the bar was fabri-cated by swaging. For comparison purposes, another bar, containing nothoria, was simultaneously prepared from the same lot of rhenium powderand fabricated by swaging. Vickers hardness measurements were takenbefore and after each working pass on the surface of each bar.

Generally, the pure rhenium maintained hardnesses below those forthe thoriated specimen, although there were some exceptions. After 70 percent total reduction, the differences in hardnesses for thoriated and un-thoriated rhenium were insignificant. The thoriated rheniuin was then drawnto 0. 016-inch wire by 10 per cent reductions between anneals. This wasdone only with difficulty, however, since the thoriated wire was much moreliable to brtýak during drawing than pure rhenium was. The thoriated wirecontained many surface impetfections, consisting mainly of longitudinalfissures. Far froanenhancing fabricabilityý it is probable that thoriaadditions reduced it.

WADC TR 54-371 Suppl 1 11

I

PHILYICAL PROPERTIES

Electrical Resist~vit

The electrical resistivity of rheniuu•i had been studied previously(l)2

but disagreement in the data and the importance of this property to theapplications of rhenium made redetermination essential. Accordingly,electrical resistivity as a function of temperature was redetermined experi-mentally in four distinct steps. First, the room-temperature resistivitywas established by two separate measurements on a length of high-purity

50-mil wire as reported in detail below. The resistivity between room tem-perature and 725 K was determined with the rhenium wire in an evacuatedtube. Heating was accomplished in a large muffle furnace and temperaturemeasured by thermocouples. The wire was mounted in the open in ahydrogen atmosphere furnace, for measurement of the resistivity from725 K to 1100 K. Temperature measurement was also by thermocouple.The same wire was also mounted in an evacuated tube for resistivity determi-nations at elevated temperatures from 1100 to 2700 K. In this case,heating was by self-resistance, and temperature was measurcd optically.

Following these measurements, the wire was sectioned and chemicallyanalyzed. It contained approximately 0. 06 per cent total impurities as shownin Table 2. A. drawing of the vacuum apparatus used for two of the fourseries of resistivity determinations is given in Figure 3. Discussion of theindividual resistivity experiments, in order of increasing temperature,follows.

Preparation of Resistivity Specimen

The rhenium metal was prepared from arnrno.ium perrhenate whichhad been ground by hand in an agate mortar to insure minimum impuritypickup. The consolidated metal was reduced by swaging and wire drawing

to a diameter of about 50 mils; and then annealed at 1750 C for 2 hours.

Resistivity•at Room Temperature

A Kelvin bridge was used for the room-temperature measurements.The resistance was measured for a length of 14. 90 cm for each test, andall readings were taken at 77 F in a constant-temperature room. About50 individual micrometer measurements were averaged to give the wire

diameter [ 0. 0518(9) inch]. The resistivity was then calculated from theresistances found on the Kelvin bridge by the well-known relationship,

IWADC TR 54-371 Suppl I 12 !1

Ia

TABLE 2. SPECTROCHEMICAL ANALYSISOF RHENIUM RESISTIVITYSPECTMEN(a)

Impurity Per Cent by Weight

Copper 0.0005

Tin 0.014

Aluminum 0.015

Silicon 0.010

Magnesium 0.005

Calcium 0.010

Molybdenum 0.004

Gold 0.0X

Total 0. 0585 (plus gold)

(Elements checked but not found: As, Sb, Ba,Be, Bi, B, Cd, Cr, Co, Ga, Ge, Pb, Mn, Nb,Ni; Pt, K, Ag, Na, Sr., Te, Ti, W: V, Zn,Zrj

(a) Analysis by W. B. Coleman & Co., Philadelphia, Pennsylvania.

WADC TR 54-371 Suppl 1 13

P"1

0C.

E ,4

oro0A 0~ zI

CLO

- 1TJ

H z

(D E

__ _ _ __ _ _ ILA

0 - 0

a)0oP;F Io Ir

U ~'~- on

oACT 431Spl11

RA~

where

p = resistivity, ohm-cm

IR = ,esistance, ohms

A = cross-sectional area, cm2

= length, cm

Resistance readings were taken at several different times following theinitial 2-hour anneal, More anneals, followed by resistivity determinations,were taken until the resistivity leveled off. A relatively level series ofreadings was reached after 3 hours of annealing; therefore the resistivityvalue.s directly after 2 hours' annealing time were discarded. Two

readings were made after 3 hours and two more after 4 hours' total

annealing time. All of these values and additional readings at 51 6, and 7hours' annealing time were then averaged, giving a calculated resistivityof 19. 50 + 0. 01 microhni-cm (at 77 F, or 25 G).

This value was plotted with the low-temperature resistivity datashown in Figure 4, and the curve extrapolated through 20 C. The resistivityvalue at 20 C was 19. 3 microhm-cm as read from the curve. This is themost accurate value of the resistivity at 20 C found in the present work.

Rfesistivity From Room Temperatureto 725 K (452 C)

The 51-miu wire used for the room-temperature ineasurements was

used also for the determinations from room temperature to 725 K, althoughchronologically the measurements from 1100 K to 2700 K reported belowwere made directly following the room temperature rmeasurements.

The variation of resistance with temperature in a 7, 1140-crn portionof the wire was measured utilizing the apparatus shown in Figure 3, ex-cept that the sight tube shown was replaced with sealed thermocouple leads.A Chromel-Alumel thermocouple was mounted so that the bead lightly touched

the center of the rheniumn wire. The apparatus was heated from 300 to 700 Kin a large muffle furnace. The resistance of the known length of wire wasireasured on,•a elvi'n brid..g, and the temperature at the wire was recordedby the Chromel-Alumel thermocouple at temperature intervals of about 25 K.The resistivity values calculated from these data are reported in graphicalform in Figure 4.

WADC TR 54-371 Suppl 1 15

SI

I.. _

N 0.! 0

41 .2 iu I ___ fb)opo

I H,- _ _ 0

-ii C0

gol UJ-W40Aj!l4S!aaI

WADC ~ ~ ~ ~ E 2R 5431S i11

Resistivity From 629K (35ý Cto 1175 K (902 C)

The range from 629 K to 1175 K is the most difficult temperaturerange to evaluate for resistivity. Since it is above the softening point oflPyrexA the vacuum-sealed apparatus of Figure 3 cannot be furnace heated.On the other hau1d, the temperature range is below the useful range ofoptical pyrometers, so that self-resistance heating with a sealed unit can-not be used. Attempts to heat by self-resistance and to measure the tem-perature by a thermoelement welded to the wire were also unsuccessful.because the current used for heating the rhenium specimen caused severeerrors in the thermocouple readings.

Finally the 51-mil specimen was removed from the vacuum-sealedPyrex tube, mounted in an open alumina boat, and fitted with the properleads for Kelvin bridge resistance measurements. Two Chromel-Alumelthermocouples were mounted so that the hot junctions touched or were invery close proximity to the rhenium wire at the center of the test portion.The apparatus was inserted into a hydrogen-atmosphere znolybdenum-woundtube furnace and resistance readings taken over the desired temperaturerange. Different resistance values were found when the leads to the rheniumprobes were reversed at the Kelvin bridge terminals. This may have beendue to a ternperature gradient in the wire, resulting in a thermocoupleeffect. To nhinirrnize this effect, two resistance readings were taken ateach temperature with the leads reversed. The resistance readings werethen averaged. Any temperature gradient existing in the wire was assumedto be uniform. Resistivity values calculated by this procedure are plottedin Figure 4, and fall on a smooth curve connecting the low-temperature andhigh-temperatuie values determined by other methods. Thus, the assump-tion that a uniform ternperature gradient existed in the wire appears to becorrect, within the accuracy limits of the plot in Figure 4.

Resistivity From 1100 K (827 C)to 2050 K (1770 C)

To determine resistivity from 1100 K to 2050 K the resistance of ameasured length of the rhenium specimen was determined from the voltageand the direct current through the measured length. In most of these meas-urements, the voltage was measured with a Leeds & Northrup Type K2potentiometer, although for a few measurements, the voltage was determinedwith an SIE Model P2 potentiometer. The current was determined by meas-uring the voltage across a Leeds & Northrup standard resistor (L & NNo. 4363) placed in series with the rhenium wire. When two Type Kzpotentiometers were employed, they were balanced simzultaneously andstandardized from the same standard cell. A schemnatic diagram nf theelectrical-measuremnent apparatus is shown in Figure 5. The temperature

WADC TR 54-371 Suppi 1 17

00

U)ZLL,a 0 .a0 a. -

0 E

70 0G 0~E)a

]4 0

II

0i y E

CL)

r_ CL 0 0WAfn F- 5437 >up 1

w co n vt

was me;asured with a Leeds & Northrup optical pyrometer. An average was

taken of several temperature readings by two observers.

The vacuum chamber and the mechanical structure of the resistivityapparatus are shown in Figurc 3. The rhenium wire was supported bymolybdenum clamps) which in turn were connected to tungsten leads. Theresistivity was ultimately determined from the voltage measurements and

from the physical dimensions for a 7. 5465-cm-long center section of the 23-cm long rhenium wire. The diameter of the wire was 0. 1316 cm. A tem-

perature gradient within the sensitivity of the optical pyrometer was not

detected at 2500 F.

From the dimensions of the wire specimen and the value of the stand-ard resistor, the resistivity is calculated from the equation:

p = 1. 8024 microhrn-centimeter

where VI is the potential in volts across the rheniurn specimen, and V? isthe potential in volts across the standard resistor. The value of the standardresistor,, as certified by the National Bureau of Standards, is 0.0010059 ohm

absolute at 25 C while passing 60 amperes. Sixty-five resistivity determ-ia-

tions were made within the stated temperature range. The spectralemissivity (at X = 0. 655yi) was taken from earlier work on this project asequal to 0. 42, over the temperature range employed. The resistivity as a

function of temperature in this temperature range is given in Figure 6.

Resistivity From 2000 K (1727 Cto 2700 K (Z4Z7 C1

For the range from 2000 K to 2700 K. the resistance of a measured

length of the rhenium specimen vas determined, as before, from the voltage

across and the d-.c current through the measured length. A battery power

source was not available that would supply sufficient d-c current for tem-

peratures above about 2000 K, so that it was necessary to employ a d-c

source consisting of a generator-battery combination. The voltage fluctua-tions of this source were, of course, greater than those in the simplebattery supply that was employed for the resistivity measurements below

2000 K. In most of these measurementsj the voltage was determined with

a Weston 1/2 per cent voltmeter, although for a few measurements a

Southwestern Industrial Company Model P-2 potentiometer was used. Thecurrent was determined by measuring the voltage across a Leeds andNortilrup standard resistor (L & N No. 4363) placed in series with therhenium wire, An L & N Type K-2 potentiometer was employed for this

measurement. The apparatus was identical with that shown in Figures 3and 5 except for the voltage source and voltmeters. The temperature wasmeasu::ed with a Leeds and Northrup optical pyrometer. Am average was

taken of several readings at each temiper.tture.

WADC TR 54-371 Suppl 1 19

1' 00

V ~P4

H- -- 0 0

-00 x -- 1-1 -- _ !40~-

WADC rR 5-371Supp 1 o

From the dimensions of the wire specimen and the value of the stand-ard resistor, the resistivity of rhenium is calculated from the equation,

n - (%,I Ad- VIp,,,. --- nmicrohm-centimeter,V -

where V1 is the potential in volts across the rhenium specimen) V? is thepotential in volts across the standard resistor, and C is a correction factor;the meaning of which is discussed below.

At the very high temperatures reported here, the evaporation of therhenium causes a noticeable change in the resistivity measurements. Thedata were corrected for the resulting change in diameter by employing theknown resistivity values for the temperature range 1100 to 2000 K, whereevaporation is not significant. The pruoedure for determining this correctionwas as follows: The rhenium wire was raised to a high temperature, andthe current through the wire and the voltage across the measured length weredetermined with as little loss of time as possible. Immediately after thesemeasurements, the temperature was dropped to below 2000 K to reduceevaporation. Several measurements were then taken in the temperaturerange from 1100 to 2000 K and a smooth curve was drawn through thesepoints. Fromr this curve and the known relationship between resistivity andtemperature within this range determined previously, the change in diam-eter was determined. This is the value of C in the equation above. Thisprocedure was repeated until a number of corrected resistivity determina-tions had been made in the temperature range from 2000 to 2700 K.

The values of resistivity as a function of temperature at the very hightemperatures are plotted in Figure 6, along with the values for temperaturesbetween 1100 and 2050 K. Scatter in the data is primarily due to voltagefluctuations in the d-c power source.

Sui- nary of the Resistivity Data

All of the electrical resistivity data at various temperatures have beenplotted on a single graph, and a resistivity vs temperature curve drawnfrom room temperature to 2700 K. This curve, without the specific datapoints so as to give greater clarity, is given in Figure 7.

In addition, a mathematical expression of the power series type hasbeen developed relating the electrical resistivity to absolute temperatureas follows:

p = -4.50 + 9. 00 x 10 2 T 2.35 x 10-5 TZ + 2.2 x 10- 9 T 3 ,

WADO TR 54-371 Suppl 1 2:1

0

I .4

1.4

olo

WAD_ oR 54313pl12

where

P = resistivity, microhm-centimeters

T = absolute temperature, K.

Figure 8 gives the raw data together with a plot of this equation fromroom temperature to 2400 C. The equation is a good expression for thetemperature-dependency of resistivity, particularly above 400 C. How-ever, some deviation occurs between room temperature and about 300 Cjand a second equation has been developed for use in this temperature rangeonly as follows:

P1 = -4.50 + 8.20 x 10--T 2.8 x 10-1 1 T 4

where

P1 = resistivity, microhm-centimeters, from 0 to 300 C

T = absolute temperature, K.

For convenience, values of resistivity have been taken from Figure 4(room temperature to 300 C) and from Figure 7 (300 C to 2400 C) andsummarized in Table 3.

Tern-perature Coefficient of Resistivity

Agte, et al. (2), has reported the temperature coefficient of resistivity,a. for rhenium from 20 C calculated for several temperatures in the rangesfrom -190 to 120 C and 2222 to 2712 C. In the present work, the tempera-ture coefficient of resistivity has been calculated for tetmperatures from 20 Cto 2300 C at 200-C intervals. The data are compared to Agte' s results inTable 4, where it can be seen that the present work gives higher valuesnear room temperature and lower values at very high temperatures.

Specific Heat of Rhenium

Relative values of the specific heat were determined for the tempera-tu~re range from 1620 to Z690 K. The specific heat of rhenium has beenmeasured up to 1473 K (1200 C) by Jaeger and Rosenbohm(4). Whenemploying the calorimetric method, they found the following empiricalrelationship between the specific heat and the temperature:

C = 0. 03256 +0 . 6625 x 10- t cal/g/C . (1)

WADC TR 54-371 Suppl 1 23

C4~ p

+ 0

10 z Jx 02

cm~4-i -~bI,, -

(Fl

- - -I -'--i-i-K ----1-*6-h

'j~i1 CL _

IN I

TABLE 3. THE ELECTRICAL RESISTIVITYOF RHENIUM METAL FROM20 TO 2400 C

Temperature, Resistivity,C microhm- c entimeters

19.3100 25.4200 32.8300 40.0400 46.5500 52.6600 58.0700 63.0800 67.0900 72.5

1000 76.51100 80.51200 84.01300 87.01400 90.01500 93.01600 96.01700 98.51800 101.01900 103.02000 105.02100 N6. 52200 108.02300 109.02400 110.0

WADf TR 54-371 Suppi 1 25

TABLE 4. TEMPERATURX COEFFICIENTS oF RESISTIVITYCALC ULATED FROM THE PRESENT WORK ANDCOMPARED TO THOSE OF AGTE, ET AL. (2)

Resi stivity- Temperatur e

Resistivit,, Coefficient, I/C(p), RT

Tempera- nicrohm- = _ 1ture, centimeters R 2 - x 103

T Present Agte_ T (a) _ 293C K Work et al. Present Work Agte, et al.

-190 83 4.94 3.65-30 243 15.4 3.35

0 Z73 19.8 3.1120 293 19.3 Z1.1 3.11

100 373 25.4 3.95120 393 26.1 3.11300 573 40.0 3.83500 773 52. 5 3. 55700 973 63.0 3.33900 1173 72.5 3.13

1100 1373 80.5 2.941300 1573 87.0 2.741500 1773 93.0 2.581700 1973 98.5 Z.441900 2173 103.0 2.312100 2373 106.5 Z. 172222 2495 125 2.232300 2573 109.0 2.042417 2690 130 2.14271Z 2985 134 1.98

(a) In degrees Kelvin.

WADC TR 54-371 Supp1 1 26

A~l

Relative values of the specific heat were determined in this laboratory

by a method whereby the heat input is increased in a step. A rhenium wire

was heated to each selected temperature by resistance heating with.a con-

duction current. The stepwise increase in heating was obtained by a step-

wise increase in the current. From the observed rate of rise of ternpera-ture and from the physical consetants for rhenium wire, the relative specific

heats at different temperatures are readily calculated. At some preselected

value of the temperature, TI, the power input per unit length to the wire,

Pl, may be related to the temperature by the expression,

P1 = fl (Ti)

If the power is increased to P 2 , thenAT

P 2 = f2 (T 1 ) + CpMe At (3)

where Cp is the specific heat, Me is mass per unit length of the wire, and

AT is the rate of rise of temperature with time. The effect of temperature

At

on radiation losses may be neglected, provided the rate of increase of tem-

perature with time is measured at the instant of the initial increase in tem-perature; i.e., as At-.- 0,

P? = f I(TI)+Cpie - ; (4)

and

P? - PI = CpMe (5)

Since the density is also unchanged at the instant the temperature starts to

increase,P2 - Pl R (12 - I2

c p 0C (6)dT dT

dt dt

where R is the resistivity of rhenium at the temperature TI, I1 is the currentthrough the wire before the step increase in current, and 12 is the sum of I1and the step increase in current.

The apparatus employed for these measurements is illustrated by the

sketch in Figure 9. The rheniumn was heated to a selected temper'ature byadjusting the conduction current with the resistor, R 1 . A step increease inthe current was applied by shorting out R,. The change in temperat-ture was

measured by the output current from the photomultiplier tube. This outputcurrent was amplified by a cathode-followcr circuit, and its output wasapplied to the d-c amplifier in the oscilloqcepe. The sweep time of the

WADC TR 54-371 Suppl 1 27

Oscilloscope trace was sufficient to allow the wire to attain the higher,stal-le te-nperatuzc; T 2 . The tirne for one sweep of the trace on theoscilloscope was approximately 4-1/z seconds: and was the same for allmeasurements. The value of the resistance R2 was adjusted to obtain a

temperature diff~erence between Tz and Ti such that TZ is stabilized before

the end of the trace. The stable temperaturesa Tl and T2. were measuredwith an optical pyrometer. The relationship betweon brightness and true

temperature of rhenium in the temperature range from 1000 to 2600 C is

Lt = 1. 2 4 tb - 2 07 , (7)

where Lt is the true temperature, C, and tb is the brightness temperature,

in degrees centigrade. This relationship is taken from, data determinedpreviously(l).

Timedelay

dT K

2 Photomultip ie-Re , -L--TV

{' Trace 1>

Cathode 1follower--J

A- 12094

FIGURE 9. DIAGRAM OF EQUIPMENT FOR SPECIFIC-

HEAT MEASUREMENTS

All oscilloscope traces of the change in temperature with time werephotographed. enlarged, and printed in order to obtabi accurate measure-

ments on the prints. The current was increased stepwise three times at

each temperature, and each stepwise increase in the conduc'tion current was

WADC TR 54-371 Suppl 1 28

photographed. A tyvical photograph is presented in Figure 10. The verticalwidth of the trace is an indication of the pickup of the stray fields in thevarious circuits. The presence of these fields docs not affect the accuracyof the measurementsj however, so shielding was not applied to reduce thestray fields to the small value that is required for a narrow trace. FromT 1 and T2 and the time scale of the oscilloscope traces, the initial value of

d-T was determined. The stepwise variation of the conduction currentdt

through the rhenium wire as a function of time is presented in Figure 11.

After measurements on all of the photographs and after reduction ofthe data, the relative values of the specific heats at different temperatureswere those given in Figure 12. Since there are three measurements foreach temperature, the vertical lines are drawn to show the range of theprobable values,, which was calculated from the three measurements at eachtemperature. The spread in the data is greater than the probable errors butthe points show a very definite trend. The temperature range of the measure-ments is from 1620 to 2690 K and is to be compared with the measurementsof Jaeger and Rosenbohmj for which the highest temperature is 1473 K.The line is drawn to obtain the best correspondence between the data fromthis work uixd the previously published measurements of Jaeger and Rosen-bohm.

The largest error in the measurements arises from the inaccuracy ofthe measurements of current. It is most unfortunate that a carbon resistor"was employed for the permanent current-varying resistor, R!, in thecurrent-pulsing circuit. Carbon resistors are notorious for variation inresistance under pulse operation,, Repeated measurements of the pulsecurrent under presumably constant conditions of operation have shown that

the current peak varies from pulse to pulse. Although the average currentfrom two or more pulseu was employed in the calculationsj the uncertaintyn-i the value of tLU current is believed to be the largest variable and to

accoimt for n-most of the variations between readings. An error in the currentis particularly serious, since the square of the current is employed in the

calculations of Cp from dT

For the purposes of comparison, the published values of the specific

heat of rhenium and of tungsten are pres..nt.ed in Figure 13. The specificheats for tungsten are from Smithells(17) and the specific heats for rheniumup to 1473 K are from the work of Jaeger and Rosenbohm(4). The dottedline that extends the data of Jaeger and Rosenbohlm to higher temnperaturesis the line from Figure 12, i. e., the experimental curve from these data.Since, on the temperature scales the measurements from this study do notoverlap those from the published work, a basis for extrapolation of the curvewas necessary. it was assurned that the magnitude of the 5pecific heat and U

WADC TR 54-371 Supp! 1 29 I

Photograph of Response of Rhenium Wireto Rise in Temperature From 2780 to2900 F

dT

/, A

*

o T

______--Time

FIGURE 10. A REPRESENTATIVE RESPONSE OF THE SPECIFIC-HEAT APPARATUS TO A STEP INCREASE I14 CURRENT

WADC TR 54-371 Suppl 1 30

Photograph of Typical Time -Cur rent Relations hip

Time IFIk-GURE 11. RELATIONSHIP BETWEEN CURRENT THROUJGH RHENIU.M

WIRE -AND TIME, SHOWING A TYPICAL STEP INCREASEOF THE CURRENTI

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the slope of the curve of specific heat versus temperature were known at1473 K (the solid line). The relative magnitude of the values of specificheat from these measurements was then adjusted until the best smoothcurve could be drawn through all of the data. The selected curve fromthese measurements is shown on a large scale in Figure 12, and the relationof the extension to the published data is indicated by the dotted line inFigure 14. The increase in specific heat in this temperature range isattributed to the contribution from electrons. It is also "worth noting thatthe high-temperature values of the electronic specific heat for twigsten...[ is] in agreement with the values calculated from the theoretical energy-level systems". (3) The energy-level systems for rhenium are not so wellknown as for tungsten., so that calculations are not feasible. From thecloseness of these metals in the atomic table, the conctihutions of electronicspecific heats to total specific heat should increase similarly at high tern-peratures for the two metals.

Vapor Pressure

The accuracy of previous determinations(l) of the vapor pressure wasquestioned on the supposition that there was an appreciable temperaturegradient in the wire. The temperature gradient of a 55-mil wire wasdetermined at 2200 C and 2550 C and found to be so small as to contributeonly an insignificant correction to the vapor pressure of rhenium as pre-viously given.

-Electrornotive Force Studies

Rhenium, either pure or alloyed,, has potentialities as a thermo-element rrmaterial because of its strength, ductility, and high melting point.No information has been reported in the literature concerning the thermalemf of pure rhenium, although rhenium additions to platinum have been dis-cussed. (6) Accordingly, the thermal emf developed by rhenium againstplatinum was measured, following which measurements were made of thethermal emf generated by Re-Mo, Re-W, and Re-Ta thermocouples.Platinum-base alloys with one and two per cent rhenium vs platinum werealso evaluated.

Rhenium vs Platinum

A 20-miu rhenium wire was welded to a 20-mil platinum wire to forma therrnxocouple 18 inches long. The test couple was surrounded by threePt-PtlORh cotpJles, and the bunudle Uf four couples inserted in a 1/2-inch--diameter closed-end protection tube in a molybdenum-wound tube furnace,

WADC TR 54-371 Suppl 1 34 ii

While protected by very slowly moving argonj readings from the threePt-PtlO..h measuring couples and the Pt-Re couple were taken. A Leedsand Northrup semiprecision potentiometer was used for recording the enifproduced.

Readings were taken from 2,0 to about 1600 C under the protection ofaArgon. This same general method was also used in taking a second set ofdata, but in the latter instance the atmosphere was hydrogen. D!4ta obtainedwere in excellent agreement with the previous measurements under argon.The three Pt-Ptl0Rh valLe's were averaged and used to establish the tem-perature for which the Pt-Re value applied.

The emf-versus-temperature data for the platinum-rhenium couplewere plotted and appear to follow the power-series type of equation,

E = 1.56- 0.90 x 10-2 T + i.29x i0-5 T2

where E is measured in millivolts and T is in degrees Kelvin. In Figure 14;the plot of this equation is compared with the experimental data.

The first derivative of the thermal emlf equation gives the thermo-electric power, P = -9. 0 + 2. 58 x 10-2 T, microvolts per degree. Thethermoelectric power of rhenium increases linearly with increasing temrperature. Thus, the Re-Pt couple has increasing sensitivity with increasingtemperature; as a thermocouple elemutu, rhenium should be most usefulat high temperatures.

Figure 15 shows the emf produced by rhenium and other metals withreference to platinum. The figure shows that the W-Mo couple, which hasbeen used in industry for many years, produces an emf of about 2 my at1600 C and only about 7 n-v at 2200 C7t . It is apparent that the Re-Mo orRe-,W thermocouples would be expected to produce an emf of about 30 m3at 1600 C. Thus Re-Mo and Re-W thermocouples might be of value forhigh-temperature measurement because of the high emf' s apparentlypossible. The promise of Re-W and Re-Mo thermocouples, as indicated inFigure 15, resulted in the studies of these and the Re-Ta thermocouplewhich follow.

Rhenium vs Tungsten• Molybdenumand Tantalum

Re-W, Re-Mo, and Re-Ta thermoco'uples were prepared by helium-atmosphere welding of beads on 20-mil wires of the coexrponent metals. Thetemperature-vs-ernf relationships of these couples were measured under avacuumn of 0. 4 to 4. 0 microns by putting the therrnocouple beads into acavity in a large self- resistance-heated rnolybdenurt rod, Temperatture of

WADC TR 54-.371 Suppl I )

281 l j- .. ___ ____

26 _/24 - !

I122 LegendI

- Plot of the equation

201 E. 1.56-0.90x O 2 T7+ 1.29xlO5T2

1 0 Experimenial data points I

20

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6

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0 200 400 600 800 1000 1200 1400 1600Te mperoture,C

A- SS20

FIGURE 14. EXPERIMENTAL POINTS AND CALCULATED CURVE OFTHE THERMOELECTRIC FORCE PRODUCED IN APLATINTYM-RREN.UM THERMOCOUPLE

WADC TR 54-371 Suppl 1 36

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the molybdenum rod was measured optically by means of a black-body hole.

The thermal emf' s generated by the three thermocouples over the tempera-ture range from 800 to 2500 C were recorded on a semiprecision potentiom-eter. The experimental data is shown in Figure 16.

The thermal emf' s generated by the Re-W and Re-Mo couples are asestimated from the Re-Pt data using published values for the Pt-W and

Pt-Mo couples. Both the Re-W and Re-Mo couples produce high emf' s,with good sensitivity to temperature, though the emf' s are not quite as highas estimated from the earlier Re-Pt work. The Re-Ta couple produced anemf with insufficient potential to be of practical use.

Attempts to fit parabolic or hyperbolic equations to the molybdenumand tungsten curves were unsuccessful, particularly because of the linearityof the Mo-Re data above 1700 C, and of the W-Re data in the ranges from800 to 1700 C and from 1700 C to 2500 C. Accordingly, linear equationswere developed for the straight-line portions of the data) and a parabolicequation for half of the molybdenum data as follows:

Molybdenum vs Rhenium 800-1700 C E = 25.8 - 18,400

T1700-2500 C E -0. 00485T + 7. 6

Tungsten vs Rhenium 800-1700 C E = 0. 0151T - 9.8

1700-2500 C E 0. 00957T + 1.3

In these equations, E is the electrometric force in millivolts and T

the absolute temperature in degrees Kelvin. The equations are plotted with

the raw data in Figure 16. The first derivative of these equations gives thethermoelectric power, P, such that

dEP= - x 103, the thermoelectric power in microvolts per degree.

Molybdenum 800-1700 C = x 106T2

1700-2500 C P = 4.85

Tungsten 800-1700 C P = 15. 1

1700-2500 C P = 9. 57

As shown by the thermoelectric power and by the slope of the curvesin Figure 16, the Re-W thermocouple produced a greater change in emf withtemperature (thermoelectric power or "sensitivity") than did the Mo-Recouple, and a greater total emf at temperatures above 1500 C. In addition,the Re-W thermocouples are potentially useful at temperatures up to about2800 C, the eutectic temperature in the system, Re-Mo thermocouples are

WADC TR 54-371 Suppl 1 38

__8w--__ --Fi E 0.097 1. ---7

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800 1000 1200 1-'400 1600 1800 2000 2200 2400 260081ack,.&Ddy Temperature, C A-i•62Z

FIGURE 16. EXPERIMENTAL DATA AND MATCHING CURVES CALCULAUTEDFOR THE THERMAL ELECTROMOTIVE FORCE PRODUCED BY

RHENIUM-MOLYBDENUM AND RHENIUM-TUNGSTEWTHERMOCO UPLES

WADC TR 54-371 Suppl 1. 39

= . . . . . . .

of interest up to about 2500 C. However, both t•ungsten and molybdenum inthermocouples will be subject to brittieness from recrystallization followingexposure to high temperatures) while the rhenium would be expected to re-rnain ductile. Any thermocouples containing tungstenj molybdenum, orrhenium would, of course, be useftml only in a neutral or reducing atmosphere,so that the deleterious oxides of these metals would not form and destroy theco upl e s.

MECHANICAL PROPErTIES

The previous technical e'eport(l) included the results of a number ofmechanical properties determinations on rhenium. Most of the propertiesreported were found for rhenium rod or rhenium wire. •n the more recentwork, additional mechanical properties have been studied on rod and wireand al!so on sheet and foil, so that a more complete picture of the mechanicalproperty behavior of rhenium can be drawn.

Tensile Properties of Cold-Rolled Sheet

Ten-rnil rhenium stripwas cold rolled frorn sintered bar) and a sectionremoved for tensile testing. The remaining strip was reduced in 10 percent stages and sections were removed until about 30 per cent total reduc-tion was reached. Tensile and hardness data were obtained for each re-ductioni step.

Tensile specimens prepared from the strip were all standard-typetensile specimens with an 0. 2-inch reduced-section width knd a 1-inch gagelength. Initial strain was measured by means of Type A-7 strain gages, IDividers were used to record extensions beyond the limit of the straiz gages.Strain rate was 0. 01 inches per minute.I

The tensile data are presented in Table 5, while the ultimate tensilestrength, 0. 2 per cent offset yield strengthj and elongation are reported ingraphical form in Figure 17 as a function of cold work. The offset yieldstrength and ultimate tensile-strength curves show a marked increase instrength with increase in cold work, as observed previously for cold-worked wire(l). The total elongation of annealed sheet was found to be 28per cent. This is the highest value found yet for rhenium, since annealedrod was found to have an elongation of about 25 per cent, and an ann.aled Iwire ihad about 10 per cent(l). At 20 to 30 per cent reduction, the elongationdrops to 2 per cent because of severe work-hardening.

WADC TR 54-371 Suppl 1 40____ __ _ __ ___ ____ __ ____ __ _ _

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FIGURE 17. TENSILE PROPERTIES OF ANNEALED AND OF COLD-ROLLED RHENIUM SHEET

WADC TR 54-371 Suppl I 4Z

Work-Hardening Characteristics of Rod Sheet and Wire I

The high work-hardening rate of swaged rhenium rod has been reportedpreviously(l). Hardness values of over 800 VHN have been attained with 30 lper cent reduction in cross-sectional area from rod with an amnealed hard-ness of about 270 VHN. Further work-hardening studies were subsequentlyconducted on drawn wire) rolled sheet, and rolled arc-melted metal.

The 10-mil cold-rolled strip discussed above was used for some ofthese work-hardening studies, The. surface hardness and cross-sectionhardness were determined at the various reduction steps; little differencewas noted between the hardness values measured in the two directions.Anotther series of hardness iieasurements was made on a 3/4-inch-diameter30-gram arc-melted button, cold rolled to a 0. 15-inch thickness. Afterannealing, various further reductions, up to 40 per cent. were taken bycold rolling, and hardness values were recorded. Wire 60 mils in diameterwas reduced by wire drawing about 20 per cent in cross-sectional area, thenan additional 20 per cent by swaging with intermediate hardness measure-ments being taken.

Figure 18 shows the work-hardening characteristics of the severaltypes o L fabricated rhem compared with those of nickel. All of the !rhenium specimens exhibit a high degree of work-hardening except theroled 10-mil sheet, which work-hardened only relatively slightly. It isnoteworthy that this material had by far the smallest dimension (7 to 10 mils) Iseparating the surfaces being worked. This dimension probably was

sufficiently large, however, not to interfere with movement of the metalgrains, which were about 0. 01 mm in diameter (0. 4 mil). As shown byFigure 18, both the cross-sectional area of the specimen and the .nethoda ofworking affect the work-hardening rate; large sections and rounds promote

Shear Modulus of Elasticity

The shear modulus of rhenium (the modulus of rigidity) was determinedat room temperature on a 50-mil wire containing 0. 75 per cent thorium(as ThO2 ) by means of a torsional pendulum. The value was found to be -Z.,, 6 , o 1. psi. In addition, Poisson' s ratio was calculated to be 0. 49,using the Young' s modulus value of 67. 5 x 106 psi previously reported.However, calculation of Poisson' s ratio from these data is very sensitive.,and "a 1 per cent crror in either the shear modulus or Young' s modulus willproduce about a. 10 per cent e-rror in Poisson's ratio. Since the calculatedPoisson' s ratio was very high, experimental determination of the value 3might be of interest.

WADCO TV. 54-371 SupplI 43

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1004

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FIGURE 13. COLD WORk&-HiARDENING CHA.1ACTERISrICS FORVARIOUS TYPES OF RHENIUM 4COMPARED WITHTHOSE OF NICKEL

WADC TR 54,-371 Suppi 1 44

I[ ,I

Modulus of Elasticity Variation With Temperature

A 1/13-inch..-diameter a:rnealed rhenium rod, 6 inches longj was usedto determine Young' s modulus from room temperature to 880 C /.1"70 F).The rod was activated in a transverse-vibration app?-.atus •' ... end. in ahelium atmosphere furnace. The drive frequencies of the roe were varieduntil resonance occurred and the mnoddt us values calculated. The drivenfrequency at resonance was 569 cycles per second. The datas plotted inFigure 19,. show that the modulus decreases with temperature in an approxi-mately linear manner over the range tested.

Stress-Rupture Properties

50-mil high-purity rhenium wizes were resistance heated to varioustemperatures while loaded in tension and the rupture times recorded. Aprotective helium atmosphere containing 2 to 5 per cent hydrogen was used,and the temperaturc was rneasured by optical means.

A preliminary series of rupture tests was conducted at 1000 C. Asshown in Table 5, the rupture time was 16. 5 hours at a stress of 40, 000 psi.These data were related to the short-time tensile daLa previously reported(M)by a tentative Larsen-Miller-type plot(i I) from which the breaking stressesfor 20-.hour rupture tests at other temperatures were estimated. Testswere then run at the various elevated temperatures, using the stresses re-corded in Table 6, which reports the rupture tirne and ductility data found.These stress-rupture data were then related to *he short-kisne elevated-temperature tensile data determined previously as shown in Figure Z0. The

best value of K was found to be 5. Figure "0 also includes scales which re-late stress to temperature for rupture times of 100 and 1000 hours.

The hot-short behavior previously found for rhenium during attemptedhot fabrication in aii does not cause extre-onc brittleness under stress-rupture conditions in a neutral atmosphere, although elongation drops to I

a value of about I to 3 per cent above 500 C. Unlike the short-time tensile Icharacteristics, elongation in stress-ruptuke tests was found to increase toover " per cent it 2000 C; quitt pobsibly increased ductility would be ex-perienced with further increase in temperature. This improved ductility atvery high temperatures is expected. Such behavior usually commences atabout the recrvstallization temperature with other metals, but seems tooccur higher with rhenium.

WADC TR 54.-371 Suppl 1 45

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WADCTR 5-371Supp 1 4

TABLE 6. STRESS-RUPTURE CHARACTERISTICS OF50-MIL RHENIUM WIRE AT 1000 C

Breaking Stress, Rupture Elongationspsi Time per cent

803000 30 secondsb 2(in 3. 1. inches)

70,000 40 seconds 2(in 3. 1 inches)

60,000 5. 0 minutes 2(inl 3. 1 inches)

50ý 000 3. 7 minutes 2(in 3. 1 inches)

40, 000 16. 5 hours 2(in 4. 6 inches)

WADO TR 54-371 Suppl 1 47

LegendI

x Tensile test

S Stress-rupture test

Numbers ore temperatures, C

1000Temperature-stress relationships for the givenrupture times

600 1000 1400 1800I O0 - h r S R ,C -1• A Ia p I - "=4 0

"" 00-hSR C -,CL I0- x Room temperature

1000__ x I_ __ _ -- I.-

1500x

1 \ I - -x 2 300

______ ______2000 - -

0 5 10 15 20 25 30

T( K+ log t) x 10-3

FIGURE ZO. PARPAMETRIC PLO-T OF TENSILE AND RUPTURE DATAFOR PURE RHENIUM METAL

WADC TB. 54-371 Suppl, 1 48

ELEGCTAkONIC PROPERTIES

Thermionic Emission of Thoriated Rhenium

The thermionic emission from a rhenium wire that contained 1 per centthorium, added as TXDz,s hz.s been reported previously(l). The emission wasseveral orders of ma-gntude lower t h .- ". at -urnl-parable temperatures. In order to determine the possibl.e value of higherthorium contents on rhenium wire, thermionic emission measurements wereconducted on a rhenium wire containing 2 per cent thorium, added as ThOr.

The apparatus employed for these measurements was essentially thesame as that employed in the earlier measuremaents. The diameter of thewire was 0. 020 inch. This wire was inserted iLto the guard-ring diodepreviously described(1). The guard-ring diode enclosure was evacuated andsealed off from the pumps after thorough outgassing. The final pressureafter- seal-off -was of the order of 10-8 mm of Hg, as indicated by a SylvaniaVG- IA rn~ometer.

The rhenium wire was initially heated for a short period at a tempera-ture of approximately 2300 G. Temperature measurements and saturationemission current for a selected temperature were obtained after the selectedtemperature had been maintained for several hours. The saturation currentswere obtained fromia the Schottky slopes in the plot of the log of the currentversus the square root of the field strength. Aia optical pyrometer wasemployed for the temperaturc measurements.

The Skhottky slopes for four temperatures are shown in Figure Z1.The saturation currents are as follows:

Temperature, Saturation Currents,K __amp/cm 2

1833 1. 65 x 10-41894 3. 73 x ,0(-41944 6.U9 x 10-4

2005 8.48 x 10-4

These values were inserted in a Richardson plot for determ•-ination of theequivalent work function (see Figure 22). For the three lowest teniperatures

the plot is a straight line. However, the value at the highest temperaturemeasured falls off this line. If a straight Line is drawn through the valuesat the three lowest temperatures, an equivalent work function of 2. 7 ev isfound with a corresponding A value of 0- 01 amp/cmZ.

WADC TR 54.-371 Suppi 1 49

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"WADO TR 54-371 Suppl 1 51

In this very short survey of the properties of thoriated rhenium,several questions were not resolved. If a complete layer of thorium was onthe rhenium surface, the expected emission would have been much higherthan was obtained here. Obviously, this condition was not met in this meas-uren.ent, and the diffusion rate of thorium in rhenium needs study. This isonly one example of the questions still facing those that work with rhenium.

To take advantage of the properties of rhenium and to satisfy the needfor high electron emission, a more thorough study of thoriated ,h--ea-h-eniumn isniedcU. Some of the points which should be included in such a study are:(1) the diffusion rate of thorium in rhenium as a function of temperature;(2) the evaporation rate of thoriu fr-o- a rhenium surface as a function oftemperature; (3) the resistance to back bombardment of thorium o i arhenium surface, (4) a study of the breakdown of ThOZ in rhenium, and(5) a study of the mechanical properties of rhenium to which ThO2 has beenadded so that an optimum amount of ThOZ can be found, consistent withgood mechanical properties and good thermionic emission.

As has been shown throughout this project, it is expected that rheniumwould be a better choice than tungsten in certain clcctron tube applications,ior example, where the presence of water vapor is unavoidable or whereriett•rs -may not be used.

The Photoelectric Threshold of Rhenium

The deterfaination of the photoelectric work function of irhenium wascompleted during this report period. Photoelectric measurements on a stripof outgassed rhenium gave a value for the work function of 00 = 4. 66 * 0. 01 evor X0 = 2662 ± 4 A when Fowler's method for determination of the thresholdwas applied to the data.

The only'previous work reported on this problem is that of Engelnan(12 )

who reported a probable value of 00 = 4. 98 volts and X0 = 2480 A. This valuewas obtained for rhenium deposited on a tungsten wire.

The thermionic work function was found to be 4. 8 ev in earlier work onthis project(1 ). Because of this and the fact that Engelman had to extrapolatehis value of the photoelectric work function, it appeared that a check ofEngleman's value would be advisable and the present work was undertaken.

A diagram of the experimental phototube used in the present work isshown in Figure 23. The rhenium, in the form of ribbon I x 5 x 0. 013 cm, jwas held by nickel supports and could be heated by passing current through it."I'A" has a nickel loop which served as the anode. The shield, "S", had asrnall rectangula. hole in it which allowed the light to hit only the rheniumon t-he cathode. ,, was a quar•z lens which focused the light on the rhenium.

WADO TR 54-371 Suppl 1 52

"T 7-A A- 18285

FIGURE Z3. EXPERIMENTAL PHOTOTUBE FOR

PHOTOELEC TRIC- THRESHOLDMEASUREMENTS

All parts of the tube were carefully cleaned before assembly. Theglass was cleaned by a conventional glass-cleaning solution, the nickel partsby electropolishing, and the rheniumn by hot concentrated hydrochloric acid.The tube was pumped for approximately 240 hours' degassing time. Thisincluded 120 hours during which the rhenium was at a temperature of 1000to 1200 C. For most of the remaining time, the tube was heated in an ovenat 450 C. The metal prts and getters wee also --heate by induction severaltimes during this period. At the end of degassing, the getters were fired undthe tube sealed off at a pressure of approximately 2 x 10-7 mm Hg while therhenium was at approxiately 1260 C. After seal-off, the rhenium washeated at 1000 to 1200 C for about 250 hours, so that the total outgassing timeon the rhenium included around Z20 hours at 1000 C and 150 hours at 1200 C.

To determine the photoelectric work function, a spectral responsecurve i ; ohtainped by rr-.ea.1ricg t-'. photocurrent per unit light intensity as afunction of the wave length of the incident light. A Hilger quarrz mnonochromr-eter was used to isolate a narrow band of wave lerig~hs. Its slit widths wereusually from 0,, 25 to 0. 5 mm. This corresponds to a maximum bandwith inthe experimental region of 1.5 to 30 A- A dia-ram of the experimentalapparatus is shown in Figure 2